L3 -5: Alkene Polymerisation

Question 1

Polyalkenes: Attractive qualities vs drawbacks

Answer 1

• Cheap monomers
• Existing production infrastructure, optimised over decades -> expensive to change
• High demand due to functionality
• Issue: Stable and resistant to degradation -> massive environmental issues (principally broken down by pyrolysis)

Question 2

3 most widespread polyalkenes:

Answer 2

• Polethylene (PE); straight chain
• Polypropene (PP); striaght chain with methyl substituent
• Polystyrene (PS); straight chain with phenyl substituent

Question 3

HDPE, LLDPE, LDPE:

Answer 3

• High density PE (no branching)
• Linear low density PE (moderate branching)
• Low density PE (high branching); lower melting point

Question 4

MAO structure:

Answer 4

• (Al(CH)3O)n
• Commonly used activator
• Cheap and effective
• As it is a polymer, crystallisation is not possible

Question 5

Typical method for termination of polymerisation:

Answer 5

• Hydrogen gas
• Protonates off the polymer chain from the metal, leaving behind a hydride

Question 6

Alternative activators to MAO:

Answer 6

• Tetrakis[3,5]-bis(trifluoromethyl)phenyl]borate anion
• Fluorinated boranes
• Aluminate salts
• Looking for weakly coordinating anions which won’t interfere with polymerisation
• Use of discrete activators allows isolation and characterisation of the active species

Question 7

What is the Cosse-Arlman Mechanism? Agostic interactions?

Answer 7

• Describes interactions which allows alkene polymerisation to proceed
• Characterised by agostic interactions (e- density in hydrogen bonds attracted to high charge density, e- deficient metal -> stabilising effect)
• Alkene should be face on to the metal; double bond should lie parallel to M-alkyl bond
• Relies on agostic interactions holding everything in the right place

Question 8

Regiochemistry in polypropylene:

Answer 8

• Position in molecule
• Depending on way the monomer approaches the metal, the polymer can have different structures
• ‘Head-to-tail’ or ‘Head-to-head’ extremes

Question 9

Stereochemistry in polypropylene:

Answer 9

• Monomer (propylene) is prochiral
• Stereocontrol is thus possible -> all tacticities are possible depending on catalyst design

Question 10

Determining Re vs Si:

Answer 10

• Prochiral
• Re face -> R (clockwise)
• Si face -> S (anti-clockwise)

Question 11

Why do C1 systems produce variable polymer products?

Answer 11

• Different sites with differing selectivities -> different selectivity depending on Re vs Si
• e.g. can produce hemi-isotactic

Question 12

Chain transfer reactions:

Answer 12

• Statistical events
• Don’t necessarily indicate termination
• Information obtained using end-group analysis